I wouldn’t say they were on the verge of defeat so much as they were already defeated. They took 99+% casualties and the rest of them could fit comfortably on the Millennium Falcon. Rise of Skywalker effectively ignores this and fills out their numbers without explanation, presumably with new recruits. But would a secondhand story about Luke really do all that much for the propaganda effort?

What makes him a symbol of hope? He came way too late, he didn’t even let them know that he was letting them escape, and they’re all going to think that he lost (which he kind of did because he died.) The reason Leia was seeking him out in TFA is so he could train new Jedi; that was the reason both sides were after him and the reason he would inspire hope. That never happened.